Field of the Invention
The present invention relates to a flow rate measurement device that measures a fluid to be measured passing through piping, for example, the amount of intake air of an internal combustion engine, and, more particularly, to a flow rate measurement device that is integrated with an environment sensor.
Description of the Background Art
Environment sensors such as a temperature measurement device, a humidity measurement device, and a pressure measurement device and control equipment are arranged, along with a flow rate measurement device that measures the amount of intake air, in an engine room of a car that uses an electronically-controlled fuel injection system. In recent years, reduction of vehicle parts assembly man-hours and wire harness simplification through integration between these environment sensors and the flow rate measurement device have been proposed.
For example, in Patent Document 1, a circuit board of a flow rate measurement device and at least one of a separate humidity measurement device and a separate pressure measurement device are assembled in a housing of the flow rate measurement device for flow rate measurement device-environment sensor integration.
In Patent Document 2, a flow rate detection element is arranged in a first sub-flow path that takes a part of intake air from a main passage, and a humidity measurement device that is mounted on a circuit board of a flow rate measurement device is arranged in a second sub-flow path that is disposed in the first sub-flow path.
In Patent Document 3, an environment sensor element is arranged in a measuring chamber that is disposed in an end portion of a supporting substrate to which a flow rate detection element and a signal processing circuit are fixed. In this example, the measuring chamber is positioned on the side opposite to the signal processing circuit across a sub-flow path where the flow rate detection element is arranged, and has a communication hole for communication with the sub-flow path.
[Patent Document 1] US Patent Publication No. 2013/0283895 A1
[Patent Document 2] Japanese Patent No. 5178388
[Patent Document 3] Japanese Patent No. 5279667
However, in the structures that are described in Patent Documents 1 to 3, integration between the flow rate measurement device and the environment sensor poses the following problems. In Patent Document 1, an environment sensor that is separate from the circuit board of the flow rate measurement device is integrated with the flow rate measurement device, and thus the number of environment sensor assembly processes increases compared to a case where the environment sensor is mounted on the circuit board of the flow rate measurement device.
In Patent Document 2, the humidity measurement device is mounted on the circuit board of the flow rate measurement device and the circuit board is electrically connected to a connector terminal through wire bonding. In a case where the humidity measurement device is soldered on the same surface as a wire bonding pad on the circuit board, a flux is attached to the pad and bonding strength between a wire and the pad declines.
The circuit board has to be cleaned with a chemical after the soldering of the humidity measurement device so as to remove the flux attached to the wire bonding pad. However, the environment sensor such as the humidity measurement device may malfunction for chemical attachment, and measures such as protection of an element portion with a mask are required. This causes an increase in the process steps.
As a method for avoiding a circuit board cleaning process for flux removal, the wire bonding pad is arranged at a sufficient distance from the electronic component such as the humidity measurement device. In this case, the circuit board has to be larger in size although flux scattering to the pad can be prevented.
In the structure that is described in Patent Document 2, the second sub-flow path may be infiltrated by fouling substances, water droplets, or the like and the environment sensor element may be stained, which may cause reduction in detection response and detection accuracy of the environment sensor element. In addition, air flow in the first sub-flow path may be disturbed since the second sub-flow path is disposed in the first sub-flow path where the flow rate detection element is arranged. This may adversely affect detection accuracy of the flow rate detection element.
In the structure that is described in Patent Document 3, an end face of the supporting substrate where the environment sensor element is arranged is exposed in the measuring chamber, and durability and reliability are required for the supporting substrate. Accordingly, a ceramic substrate has to be used for the supporting substrate, and cost reduction is impeded. Although the environment sensor element, the flow rate detection element, and the signal processing circuit are arranged on the same surface of the supporting substrate, the environment sensor element, the flow rate detection element, and the signal processing circuit cannot be arranged in close proximity, and thus the supporting substrate has to have a large size.